The present invention relates to an image forming method for developing a latent image by use of a magnetic force and an apparatus therefor.
In a copier, printer, facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus, a latent image is formed on an image carrier in accordance with image data. The image carrier is implemented as a photoconductive drum or belt by way of example. A developing unit develops the latent image with toner to thereby form a corresponding toner image.
The developing unit uses either one of a one-ingredient type developer, or toner, and a two-ingredient type developer or toner and magnetic carrier mixture. The two-ingredient type developer allows the charge of the toner to be controlled more easily than the one-ingredient type developer and causes a minimum of-cohesion to occur in the toner. With the two-ingredient type developer therefore, it is possible to execute effective control over the migration of the toner by using, e.g., a bias electric field. Further, the toner of this type of developer does not have to contain a magnetic material or contains only a minimum amount of magnetic material for obviating blurring. Therefore, a color toner in particular insures a clear color. Moreover, in the case of a magnet brush developing method that causes a developer layer to rub the surface of an image carrier, a magnet brush easily rises and desirably rubs itself against the above surface. The two-ingredient type of developer with such advantages is often used despite that the toner content of the developer must be controlled.
However, a problem with the developing unit using the two-ingredient type developer is that a single-dot line formed in the direction perpendicular to the direction of paper conveyance becomes thinner than a single-dot line formed in the direction of paper conveyance. This phenomenon will be referred to as the thinning of a horizontal line hereinafter. Another problem is that the trailing edge of, e.g., a halftone image is lowered in density or not developed at all. Let this phenomenon be referred to as the omission of a trailing edge hereinafter. To solve these problems, there has been proposed to position the main pole angle of a magnet roller at an upstream side or to set up a preselected relation between the distance between a doctor blade and a developing sleeve and the distance between a photoconductive drum and the developing sleeve, as taught in, e.g., Japanese Patent Laid-Open Publication No. 7-140730. The prerequisites with this kind of scheme are as follows:
(1) The main pole for development is positioned in a range of from 5xc2x0 to 20xc2x0 upstream of a position where the developing sleeve and photoconductive drum are closest to each other in a direction of developer conveyance (closest position hereinafter);
(2) The doctor blade and developer carrier are spaced by a distance (Hcut) of 0.25 mm to 0.75 mm;
(3) A nip for development extends over 0.30 mm to 0.80 mm (Dsd);
(4) A ratio Dsd/Hcut is greater than 1.20, but smaller than 1.60; and
(5) A ratio of the moving speed Vs of the developer carrier to the moving speed Vp of the image carrier (Vs/Vp) is greater than or equal to 1.0, but smaller than or equal to 3.0.
It is generally accepted that if the above conditions (1) through (5) are satisfied, a toner layer is protected from disturbance in halftone and solid portions when the apparatus is operated in a high-speed range. This allows a clear-cut image to be produced without any breakage of thin lines and with high and uniform density.
There is a keen demand for an improvement in the developing ability of the apparatus using the two-ingredient type developer. In this respect, Japanese Patent Publication No. 2-59995, for example, proposes to position a magnetic pole adjoining the main pole closer to the main pole. This document teaches that such a position of the magnetic pole lowers the density of horizontal lines, i.e., the thinning of a horizontal line, but the lower density can be coped with if the saturation magnetization of the carrier is lowered to weaken the magnet brush. Japanese Patent Laid-Open Publication No. 6-149063 discloses a non-contact type developing device using the two-ingredient type developer and having a pole arrangement that maintains a magnet brush spaced from a photoconductive element. The prerequisites with this pole arrangement are as follows:
(1) A developing position is defined between a pair of N and S poles;
(2) The angle between the N and S poles is between 40xc2x0 and 70xc2x0 while each flux density is 500 or above; and
(3) A magnet angle between a point where an image forming body and a magnet brush roll are closest to each other and the center between the poles is between 0xc2x0 and one-tenth of the above angle between the N and S poles, and the developing position is between the poles of the magnet.
The document describes that if the above conditions (1) through (3) are satisfied, a stable, high quality image is achievable with a minimum of blurring ascribable to the deposition of the carrier on the image forming body and a minimum of omission of an image around portions where the carrier is deposited.
In accordance with the above-described Laid-Open Publication No. 7-140730, the ratio Dsd/Hcut is confined in the range of 1.2 less than Dsd/Hcut less than 1.6. The problem with this scheme is that as the ratio Dsd/Hcut increases from 1, i.e., as Hcut decreases relative to Dsd, the magnet brush decreases in density in the closest position of the developing sleeve and photoconductive element. As a result, the magnet brush fails to uniformly contact the photoconductive element and cannot rub the entire surface of the element. This leads to an occurrence that part of solitary dots forming an image (e.g. dots sized 600 dpi (dots per inch) and spaced from each other by five to ten pixels) is reduced in size or practically omitted. When solitary dots are not uniformly reproduced, the reproducibility and tonality of a high contrast portion are deteriorated. Further, a halftone image whose density is about 0.3 to about 0.8 (ID) appears granular due to the non-uniform contact of the magnet brush.
The scheme taught in Publication No. 2-59995 mentioned earlier has a drawback that when the saturation magnetization of the carrier is lowered, so-called carrier deposition is aggravated. When the amount of charge to deposit on the toner is reduced in order to obviate carrier deposition, the amount of uncharged toner increases and brings about background contamination.
The implementation taught in Laid-Open Publication No. 6-149063 also mentioned earlier has a problem that the electric field for development is weak due to non-contact development, making it difficult to improve the developing ability.
By a series of experiments, we found that the thinning of a horizontal line and the omission of a trailing edge were presumably ascribable to the same cause. As the developer on the developing sleeve approaches the closest position of the sleeve and photoconductive element, it forms the magnet brush and is smashed by the sleeve and the element. In a conventional image forming apparatus, the magnet brush is again formed after it has moved away from the above closest position (downstream of the closest position) and is again caused to contact the photoconductive element. This magnet brush is formed by the magnetic field around the skirt of the main pole, i.e., the pole for development. On the other hand, when the magnet brush faces the background or white portion of the photoconductive element, toner in the magnet brush is biased toward the developing sleeve by a magnetic field corresponding to the background potential. As a result, the toner density at the tip of the magnet brush is lowered. For the development using the toner and magnetic carrier mixture, the developing sleeve is rotated at a peripheral speed 1.5 times to 2.5 times as high as the peripheral speed of the photoconductive element. Consequently, the magnet brush whose toner density is lowered at the tip contacts the trailing edge and single dot, horizontal lines of an image.
So long as the magnet brush mentioned above contacts the photoconductive element at the closest position of the element and developing sleeve, the toner deposited on the photoconductive element does not return to the magnet brush. This is presumably because the electric field is most intense at the closest position and allows even the toner biased toward the developing sleeve to contribute to development. By contrast, assume that the magnet brush whose toner density is lowered at the tip, as stated above, contacts the photoconductive element at the side downstream of the closest position. Then, because the electric field at such a position is weaker than at the closest position, part of the toner deposited on the photoconductive element returns to the magnet brush. In the region downstream of the closest position where the distance between the developing sleeve and the photoconductive element sequentially increases, the force tending to separate the toner of the magnet brush from the carrier and cause it to deposit on the photoconductive element sequentially decreases. As the above distance further increases, it becomes practically impossible to separate the toner from the carrier. This, coupled with the previously stated cause, causes the toner deposited on the closest position of the photoconductive element to return to the magnet brush. This reduces the amount of toner to deposit on horizontal lines and the trailing edge of an image, resulting in the thinning of a horizontal line and the omission of a trailing edge.
The present invention prevents the toner from returning from the photoconductive element to the magnet brush. Specifically, in accordance with the present invention, an electric field formed between the photoconductive element and the developing sleeve causes the magnet brush to fall or collapse (not contacting the photoconductive element) within a range in which the electric field is more intense than one capable of separating the toner and carrier from each other. Therefore, even if the toner deposited on the photoconductive element returns to the magnet brush at the side downstream portion of the developing region, the present invention makes up for the return with the toner existing in the magnet brush. This is because the electric field between the photoconductive element and the developing sleeve in the above range is more intense than one capable of separating the toner and carrier from each other. The present invention therefore obviates the thinning of a horizontal line and the omission of a trailing edge.
Further, in accordance with the present invention, an electric field formed between the photoconductive element and the developing sleeve causes the magnet brush to rise within a range in which the electric field is more intense than one capable of separating the toner and carrier from each other. In this condition, the toner in the magnet brush easily moves and insures a high developing ability. More specifically, at a position where the magnet brush collapses, the developer is packed and therefore dense to thereby prevent the toner existing therein from sharply responding to the electric field. By contrast, the present invention promotes the easy movement of the toner and maintains the developing ability relatively high. It was experimentally found that when the magnet brush rose at a position close to the closest position, a high developing ability was achieved.
In the developing region, the electric field formed between the photoconductive element and the developing sleeve causes the magnet brush to rise or fall only within the range in which the electric field is more intense than one capable of separating the toner and carrier from each other. Therefore, even if the toner deposited on the photoconductive element returns to the magnet brush at the side downstream portion of the developing region, the present invention makes up for the return with the toner existing in the magnet brush. The present invention therefore obviates the thinning of a horizontal line and the omission of a trailing edge. Further, even at the upstream side of the developing region, the range over which the magnet brush contacts the photoconductive element is limited, the toner in the magnet brush is prevented from depositing on the photoconductive element without regard to the electric field, obviating background contamination. Because the magnet brush falls only within the above particular range, the present invention is practicable even when the half center angle of the magnet roller cannot be reduced due to limitations on the magnet roller, e.g., because of a limited space available for the magnet roller.
Technologies relating to the present invention are also in disclosed in, e.g., Japanese Patent Laid-Open Publication No. 5-303284.
It is therefore an object of the present invention to provide an image forming method capable of obviating the thinning of a horizontal line and the omission of a trailing edge, the omission of solitary dots and the granularity of a halftone image ascribable to the irregular contact of a magnet brush, and the carrier deposition and therefore maintaining a high developing ability, and an apparatus for practicing the same.
In accordance with the present invention, in an image forming method using a magnet filed generating device fixed in place within a developer carrier, which conveys a developer consisting of toner and magnetic carrier and deposited thereon, for forming a magnet brush on the developer carrier, the magnet brush rubbing an image carrier to thereby develop a latent image formed on the image carrier, a magnetic field that causes the magnet brush to rise, contact the image carrier and then fall is formed between the image carrier and the developer carrier within a range in which the magnetic field is more intense than an electric field capable of separating the toner and carrier from each other.
Also, in accordance with the present invention, an image forming apparatus includes an image carrier, a developer carrier for conveying a developer consisting of toner and magnetic carrier and deposited thereon, and a magnetic field generating device fixed in place within the developer carrier and configured to form a magnetic field that forms a magnet brush on the developer carrier and causes the magnet brush to rub the image carrier for thereby developing a latent image formed on the image carrier. The magnetic field, which causes the magnet brush to rise, contact the image carrier and then fall, is formed between the image carrier and the developer carrier within a range in which the magnetic field is more intense than an electric field capable of separating the toner and carrier from each other.